ENH: Support slack variables (lower bound emulation)

[randolf-scholz]

Describe the Bug

Context: python/typing#674

Slack variables would allow us to emulate lower bounds on type variables, which is relevant for methods like dict.__or__ or list concatenation, when an outer context is present.

from typing import Never, assert_type, reveal_type

class Vec[T]:  # invariant in T
    def _items(self) -> list[T]: ...
    def _append(self, item: T) -> None: ...

def concat_vecs[T1, T2, _T_slack=Never](
    left: Vec[T1],
    right: Vec[T2],
) -> Vec[T1 | T2 | _T_slack]:  # <-- Slack variable makes return a supertype of T1 | T2
    return Vec()

class A: ...
class B: ...
class C: ...

def test_vec_concat(left: Vec[A], right: Vec[B]) -> None:
    # without outer context, the type is precise
    assert_type(concat_vecs(left, right), Vec[A | B])  # ✅️
    # outer context allows widening due to slack variables
    _0: Vec[A | B]     = concat_vecs(left, right) # ❌️
    _1: Vec[A | B | C] = concat_vecs(left, right) # ❌️
    _2: Vec[object]    = concat_vecs(left, right) # ❌️

mypy fails to find existing solutions on some of these function calls:

• _0: (T1=A, T2=B, _T_slack=Never)
• _1: (T1=A, T2=B, _T_slack=C)
• _2: (T1=A, T2=B, _T_slack=object)

21:38: error: Argument 1 to "concat_vecs" has incompatible type "Vec[A]"; expected "Vec[A | B]"  [arg-type]
21:44: error: Argument 2 to "concat_vecs" has incompatible type "Vec[B]"; expected "Vec[A | B]"  [arg-type]
22:38: error: Argument 1 to "concat_vecs" has incompatible type "Vec[A]"; expected "Vec[A | B | C]"  [arg-type]
22:44: error: Argument 2 to "concat_vecs" has incompatible type "Vec[B]"; expected "Vec[A | B | C]"  [arg-type]
23:38: error: Argument 1 to "concat_vecs" has incompatible type "Vec[A]"; expected "Vec[object]"  [arg-type]
23:44: error: Argument 2 to "concat_vecs" has incompatible type "Vec[B]"; expected "Vec[object]"  [arg-type]

no playground link as typevar defaults are still not supported.